Multi-work function cathode



April 5, 1966 B. KUHL ,2

MULTI-WORK FUNCT ION CATHODE Filed Dec 26, 1961 INVENTOR.

Bernhard Kiihl ATTORNEY United States Patent 6 Claims. 61. s1s s11 The present invention relates to sintered electrodes for gaseous and/or vapor discharge lamps, and more particularly to high pressure discharge lamps or devices, the sintered electrode body of which contains at least one of the oxides of barium, calcium, thorium and at least one high-melting metal, as tungsten, molybdenum and the like.

In the prior art, so-called storage cathodes are known which consist of a porous sintered body of a metal of high melting point in which oxides of the alkaline earth metals and of aluminum are embedded. Besides, cathodes for electric discharge devices are known in which a porous sintered body of refractory metal, such as tungsten or molybdenum, is interspersed with emission-stimulating substances and in which the cathode body consists alternately of layers of thorium compounds and refractory metals and is provided with a covering layer of tungsten or molybdenum. Such electrodes constitute a storage cathode in which, within a porous covering, the percentage of emission-stimulating substance is not limited. All of these well-known electrodes are porous storage cathodes which require either too high igniting voltage, especially at low ambient temperatures, or which in operation of the lamp tend toa very early blackening of the discharge device envelope due to strong sputtering.

In its broad aspect, the present invention is directed to provision of an electrode of such a construction and produced in such a manner that in the use thereof the functions occurring in the electrode on ignition and those during operation are separated.

The invention proposes a cathode having distinct zones differing in vaporization speed and in electronic Work function.

It is also an object of the invention to provide a cathode of greater life span than heretofore obtained.

Corollary to the above, the invention accomplishes a much slower depletion of the emission characteristic of the cathode than with prior art cathodes.

Another object of the invention is to both delay and diminish blackening of the envelope in which the cathode is employed.

An important object of the invention is to lower the required starting voltage for striking the arc to the cathode.

It is also an attained object of the invention to obtain a silent-burning arc.

In conjunction with the last mentioned object, the inice sure mercury lamp as an example of an electric discharge device utilizing the improved electrode of the present invention; and

FIGURES 2, 3 and 4 are longitudinal sections, very much enlarged from actual size, of electrodes embodying the invention, and respectively showing three forms thereof which may be employed in accordance with my concept.

As is well known, for starting an emitter, a low electronic work function is required, which means an emitter of good emitting power is needed, whereas during operation an emitter is required which will withstand high temperature, and this involves a need for low vaporization speed of the emitter constituents even if they have a higher electronic work function. With a sintered body that is homogeneous throughout its entire volume and over its entire surface, such a difierent function at starting and on the other hand during operation in gaseous and/or metal vapor discharge devices or lamps, cannot be obtained. Coil electrodes impregnated with an emitter will start well in most cases, but they show, however, a strong vaporization of the emitter during operation of the device or lump and blacken the bulb or envelope too quickly. Fundamentally, the present invention is directed to pr vision of a cathode structure overcoming the above-mentioned difiiculties and short-comings of prior art cathodes.

In the specific embodiment of the invention illustrated in said drawing, FIGURE 1 shows a general example of a discharge device or lamp designated by numeral 1, coaxially within which, near opposite ends thereof, are electrodes 2 in opposed coaxial alignment. Core pins 4 of high melting temperature metal, such as tungsten or molybdenum, coaxially within and forming part of said electrodes, project from each into the stem seals of the lamp envelope or bulb for supporting the electrodes and for making electrical connection to the usual inleads for the lamp by interposed molybdenum foils 3 likewise sealed in the lamp stems. The space within the lamp between the electrodes is known as the discharge space. In addition to said core pin 4, each electrode comprises two joined bodies 5, 6 having ends juxtaposed to each other and in the final state of fabrication are sintered together at said ends. The other ends of said bodies are remote from each other in axial direction, so that the far end of one body 5 faces its respective lamp stem whereas the far end of the other body 6 faces toward the discharge space and therefore toward vention obtains a cathode which avoids rectifier effect on starting the arc in the lamp or device in which used.

A further object of the invention is to accomplish operation with a low D.C. percentage and a lower H.F. interference from previously used electrodes. I

Another object is to provide a structure which is readily and economically manufactured.

Other objects, advantages and structural and operational features of the invention will become apparent to persons skilled in the art to which it appertains as the description proceeds, both by direct recitation thereof and by implication from the context.

Referring to the accompanying drawing, in which like numerals of reference indicate similar parts throughout the several views;

FIGURE 1 shows, in longitudinal sect-ion, a high presthe other electrode. These said bodies constitute respectively what will be descriptively termed a starting zone 5 and an operating zone 6 for an electric are produced between the two electrodes in operation of the lamp. The materials composing the two said bodies differ in certain respects from each other and the electrodes are formed :by compression and sintering of those materials all of which will be explained in greater detail in the description given herebelow.

The materials used for constituting the said bodies 5, 6, and arrangement of said bodies are such that the two zones, exposed at the exterior of the electrode, are provided having different vaporization speed and different electronic work function one from the other. The sintered body zones, at least the areas thereof which are exposed at the surfaces of said bodies, possess homogeneous emission mixtures. Body 6, located toward the discharge space where the electrodes have temperatures of 1800 to 2300 C., is composed of materials providing a more temperature-proof zone of higher electronic work function than the more remote zone 5. Said body 5, which is at that part of the electrode more remote from or turned away from the discharge space, is composed of materials having a lower electronic work function, that zone only being subjected to temperatures of 1050 3 to 1300" C. electrodes contain several peripherally superposed layers in order to give the outer layer a very low electronic work function, whereas the electrode according to the present invention contains adjacent exposed zones of different vaporization Speeds and work functions of the emitter constituents on the exposed surfaces thereof, so that the different functions of the electrode -in the gas 'cli'schargestarting and operationmay be obtained.

The different electrode zones may be arranged as shown in FIG. 2 in such a manner that the sintered body zone of lower electronic work function is located and exposed as a cylinder more remote in axial direction from the discharge area by being axially behind a similar, though longer, cylinder body zone 6 of higher electronic work function. Since zone 5 has an emission mixture of low electronic work function, it has higher emissive power than zone 6 beyond it, and will very effectively function as the starting zone, and during and a short time after starting, the arc appends thereto. Operating zone 6 is located in the lamp axially in front of starting zone 5 and is composed of a more tempera'ture-proof emission mixture, has a lower vaporization speed and has a higher electronic work function. It will be appreciated that the arc appends initially to the starting zone 5 upon instigation when the pressure is low because of its low gradient to that place where the cathode voltage drop is the lowest, which is at zone 5. When the gradient increases with the pressure, the arc jumps over to operating zone 6 because the sum of voltage drop in the arc plus cathode and anode drop becomes smaller. I

Considering now the structure shown in FIG. 3, again there is a starting zone 5 of an emission mixture of lower electronic work function. As in the previously described construction, the forward end of the body of zone 5 is blocked off by surface contact with the rear end of the juxtaposed body of operating zone 6. In this instance, the periphery of the starting zone body is surrounded peripherally by an annular apron extension formed as part of the body of said operating Zone, said starting zone body 5 having a smaller diameter than the operating zone body so as to be accommodated within said annular sleeve. The end of the start ing zone body 5 toward the lamp bulb stem is exposed, and consequently the arc may be initiated thereat since the same conditions of pressure and low gradient explained above likewise prevail. Manufacture of the dual electrode body of FIG. 3 may be performed, if desired, by first shaping and compressing the material composing operating zone body with its integral annular apron by use of a female mold andmale plunger, and then replacing the plunger with a filling of material in the formed apron to compose the starting electrode pressed therein. The formed electrode is then sintered.

As another mode of manufacture of the construction of FIG. 3, the starting zone body may be performed under pressure in its cylindrical shape, and, if preferred, may also be pre-sintered before assembly with the other body zone 6. The preformed starting zone body 5 will be introduced into the mold containing the material composing the said other body zone 5, which is appropriately compressed and then the complete electrode sintered.

FIGURE 4 illustrates the resultant electrode wherein, before inserting, there has been an intermingling of the constituent materials of the two zones 5 and 6 at the region 5/6 of meetingof said zones in forming the electrode. This intermingling is occasioned when introduc ing the respective materials into the forming mold.

It may be here noted that in all forms shown, the electrode periphery is a true cylinder having a constant diameter throughout its length. In FIGURES 2 and 4, the starting electrode zone 5 is exposed as part of the length of the cylindrical peripheral surface of'the electrode, whereas in FIG. 3, the starting electrode Zone 5 In the prior art, some part of the sintered is exposed only at its end. In all showings, the core pin enters the starting zone body 5 contiguous to the exposed end of said starting zone. The end of the electrode core pin toward the discharge space, projects, suitably, into the operating zone 6 of higher electronic work function, and said pin end may be covered by a thin layer of the material of that zone,-as shown in FIGS. 2 and 3, but if desired, said pin end may be exposed t-her'ethrough, as shown in FIG. 4, although it should not project beyond the flush condition'illustrated more than an amountequal to the length of the core diameter. In all showings, the core pin 4 is coaxial-1y within the two zone bodies 5, 6, and has a diameter proportioned to the diameter of the electrode periphery such that it is within the range of 1 to 2 up to l to '5. As a specific instance, in a fifty watt high pressure mercury lamp,

the pin appropriately has a diameter of 0.6 mm. and the electrode periphery has a diameter of 2 mm. In lamps of other wattages, the dimensions will of course be different from the ones just mentioned.

The discharge are strikes at its appendage surface Where the lowest possible voltage drop appears, and this will be at or near the electrode pin because the sintered body itself has a relatively high resistance. The outer dimensions of the sintered bodies are, suitably, such that the temperature thereof in the vicinity of operating arc appendage amounts to about 1800-2300 C. and in the starting zone remote from the operating arc appendage, to about 1050-1300 C. The proportion of the diameter to the length of the sintered body is as low as practical, since a short body 5, 6 has a short starting time.

As the active material of the emission ingredients, a compound of 40-70% and prefer-ably 65% by weight of thorium oxide, ThO 10-40% and preferably 25.5% by weight of barium carbonate, BaCO 5-20% and preferably 8.5% by weight of calcium carbonate, CaCQ and 05-50% and preferably 1.0% by weight of finely divided silicon dioxide, SiO has proven favorable. Then to form the body "of lower electronic work function for zone 5 a mixture is made of said compound with tun'g sten in proportion by weight of 60% to 40% respectively, whereas for making the body of higher electronic work function of zone 6, a mixture is made in proportion of 20% to of the compound and tungsten powder respectively.

Another favorable emission mixture for the sintered body starting body Zone 5 of low electronic work func-' tion has been obtained by admixing 60-95%, preferably 80% by weight, of tungsten powder to 5-40% preferably 20% byweight of a compound comprising approximate preferred amounts of 25% of barium oxide, 9% of calcium oxide, 65% of thorium oxide, and 1% of silicon monoxide.

In any event, a higher percentage of tungsten powder is admixed to the emission compound" for the sintered body zone 6 of higher electronic work function than to the sintered body zone 5 of lower electronic work function according to the present disclosure. A favor able emission mixture for the sintered body operating zone 6 of higher electronic work function may comprise 2-30% and preferably v by weight of 5BaO-2Al O with 70-98% and preferably by weight, of tungsten powder, with respect to which the percentage of aluminum is in the range of 0.05% to 1.0%, preferably .1% by weight. A further decrease in blackening of the discharge device envelope may be attained by further admixing 0.05-3.0% by weight, preferably 1.0% of Z rO Another favorable emission mixture for the sintered body zone 6 of higher electronic work function may consist of admixing 60-70% of the aforementioned compound of barium oxide, calcium oxide, thorium oxide and finely divided silicon monoxide with 30-40% by weight of tungsten powder.

The sintered body is preferably compressed with said core pin 4 located in place coaxially therein. The presing power is chosen in amount such that the pressed body does not fall asunder, but, on the other hand, without making the compressed body too compact. Pressures of 1000-5000 kg./cm. preferably about 3000 kg./cm. have been used successfully. After the body has been compressed, it is sintered, using a temperature in the range of 2100" to 2300 C.

The emission ingredients are used up slowly, and are, therefore, sufiicient for many thousands of startings and operating hours, and for the extent of such use the discharge envelope or bulb is blackened very little. Starting voltage with the electrodes of this invention is lower than with other known discharge lamps. On starting the arc in the lamp of the herein disclosed lamp, no rectifier effect occurs. The discharge burns after starting in a silent are; there are brought about a low D.C. percentage only and lower H.F. interference than with generally used electrodes. The costs of manufacturing sintered electrodes according to this invention are, in production of large quantities of the lamps, lower than for electrodes known heretofore.

I claim:

:1. A sintered electrode of the character described, comprising two exposed electronic emissive sintered zones a first one of which has a higher electronic work function than the second one and the second one has a lower electronic work function than the first, and said first sintered zone comprising a mixture of 5BaO-2Al O with powdered tungsten in the range of 70-98% by weight, and in which the percentage of Al is in the range of 0.05-1.0% by weight.

2. A sintered electrode of the character described, comprising two exposed electronic emissive sintered zones a first one of which has a higher electronic work function than the second one and the second one has a lower electronic work function than the first, and said first sintered zone of higher electronic work function comprising a compound of barium oxide, calcium oxide, thorium oxide and finely divided silicon monoxide, said compound being mixed with tungsten powder in the range of 60-95% by weight.

3. A sintered electrode of the character described, comprising two exposed electronic 'em-issive sintered zones a first one of which has a higher electronic work function than the second one and the second one has a lower electronic work function than the first, and said second sintered zone of lower electronic work function comprising a compound of barium oxide, calcium oxide, thorium oxide and finely divided silicon monoxide, said compound being mixed with tungsten powder in the range of 30-40% by weight.

4. An electrode of the character described comprising a plural body of exposed electronic emissive sintered zones of different vaporization speed and of different electronic work function, each of said sintered zones having as active ingredients but with different proportions thereof a compound of thorium oxide in the range of 40-70% by weight, barium carbonate in the range of 10-40% by weight, calcium carbonate in the range of 5-20% by weight, and silicon dioxide in the range of 0.055.0% by weight.

5. An electrode in accordance with claim 4, wherein said compound is mixed with tungsten powder and with a greater proportion of said tungsten powder in the mixture for one sintered zone than in the mixture tor the other sintered zone.

6. A sintered electrode for electric discharge devices more particularly high pressure discharge lamps having an envelope with discharge space therein; said electrode comprising a sintered body containing at least a substance selected from the group consisting of the oxide-s of barium, calcium and thorium and at least one high-melting metal; a core pin of high-melting metal in said body; and said body having exposed electron emiss'i-ve sintered zones of different vaporization speed and of different electronic work function; the one sintered zone having lower vaporization speed has the higher electronic work function and is located toward the end of the electrode most proximate to the discharge space, whereas the second sintered zone having higher vaporization speed has lower electronic work tunction and is located toward the other end of the electrode more remote in axial direction from the discharge space; one of said sintered zones containing a greater proportion of high-melting metal than an adjacent sintered zone, the sintered zone located more proximate to the discharge space being the one containing said greater proportion of high-melting metal than the other sintered zone; and said sintered zones having a contiguous region of intermingled constituents.

References Cited by the Examiner UNITED STATES PATENTS 1,520,794 12/1924 Zons 176 1,922,244 8/ 1933 Hunter 31537 2,185,410 1/1-940 Lederer 75-176 2,202,108 5/ 1940 Laise 75176 2,460,739 2/ 1949 Francis 3 13346 2,473,550 6/ 1949 Spencer 313346 2,492,142 12/ 1949 Germeshausen 313346 2,886,7 7 5/1959 Fruengel 313-311 X OTHER REFERENCES Publication: Tungsten, by Li and Wang, second edition, Reinhold Publishing Corp, New York, 1947.

GEORGE 'N. WESTBY, Primary Examiner.

BENNETT G. MILLER, Examiner.

L. D. BULLION, K. OROSSON, D. E. SRAGOW,

Assistant Examiners. 

1. A SINTERED ELECTRODE OF THE CHARACTER DESCRIBED, COMPRISING TWO EXPOSED ELECTRONIC EMISSIVE SINTERED ZONES A FIRST ONE OF WHICH HAS A HIGHER ELECTRONIC WORK FUNCTION THAN THE SECOND ONE AND THE SECOND ONE HAS A LOWER ELECTRONIC WORK FUNCTION THAN THE FIRST, AND SAID FIRST SINTERED ZONE COMPRISING A MIXTURE OF 5BAO$2AL2O3 WITH POWDERED TUNGSTEN IN THE RANGE OF 70-98% BY WEIGHT, AND IN WHICH THE PERCENTAGE OF AL IS IN THE RANGE OF 0.05-1.0% BY WEIGHT. 